LESSONS FROM THE WAR.
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The Evolution of the Ideal Commercial Motor. From a Designer's Point of View.
IN THE FIRST part of the brief survey of the subject as to what, speaking from the technical point of view, we have learned from the war, the writer dealt with the old-time problem of the live-axle versus chain, drive, with the question of the. best method of transmitting the thrust and torque reaction from a live axle, with the merits and demerits of flexible ring universal joints, with cooling systems and other matters upon which it has been possible to ,establish useful general deductions from the valuable
data which have become available in the past three years or more.
War-time service has provided conditions of a strenuous nature, which have placed great strains upon frames, and the experience gained has been invaluable because the frame being the foundation of the whole chassis, any weakness or failure here is bound to have re.sults which may be far-reaching and disas
trous in their effect on other portions of the mechanism of the car.
One point which, perhaps, more than anything else calls for improvement is the method of fastening the various membeIs and components, such as spring brackets, etc., to the frame. In many eases rivet a are used for this purpose, it being erroneously taken for granted by some designers that this is the best method of making a sound job.
Experience, however, points to the conclusion that, in any case where excessive vibration has to be met, there is nothing so good as a properly-fitted bolt, and, in numerous cases, rivets which have worked loose are being replaced in this way. Unless a rivet is closed in such a way that the hole is perfectly filled, it is only a question of time before that rivet ceases to perform its proper function.
It is, of course, impossible to examine the condition of the shank of a rivet after it is fitted : if it could be examined, it is probable that, in 90 per cent, of cases, a state of affairs as shown. in Fig. 1 (A) would be disclosed. This is more or less unavoidable with the usual system. of hot riveting, and, as the parts arc merely held together by the contraction of the rivet in cooling, after a certain period of running, some slight movement takes place which leads to a graduallyincreasing amount of slackness in the joint. On the other hand, in the case of a properly-fitted bolt, the bole is accurately filled Fig. 1 (B), Qud, provides' the nut is not allowed to slack off, no initial movement and, therefore, no wear can take place.
In addition, rivets are, in many cases, so inconveniently placed that they are not sufficiently accessible to enable a good job to be made by the riveter. •
The writer is of the opinion that the best results would be obtained if rivets were altogether discarded in frame construction, an additional advantage being the facilitation of replacements of broken spring brackets, frame members, etc. With regard to the general design of the frame, all the evidence points to the superiority of an under frame, mounted on the three-point principle, carrying the engine and gearbox. In this way only can permanent alignment between these two units besecured under all conditions.
Separate three-point suspension of each on a rigid frame is not in itself sufficient, since, in the event of a.0 a badly-twisted frame, c sed, for instance, by one
wheekfalling into a hole in the road, the crankshaft and gearbox are no longer in line, with the result that, unless a proper double universal .joint is provided behind the clutch, trouble in this direction will occur, and, what is perhaps more important, if the hand starting gear be fixed to a cross member of the frame, it will be thrown out of line and become temporarily useless. The above argument will be more clearly understood by refereuce to Fig 2. Side members should be constructed of channel-. section steel, preferably straight in plan, i.e.' not inswept at the front end., as is done in some designs, and of sufficiently deep section to give the requisite rigidity without truss rods. These latter invariably lead to trouble sooner or later, owing to vibration. A small but important fault has, in some eases, been brought to light in connection with the front axle swivel pins, on those vehreles which are provided with a ball thrust bearing at this point, for the purpose of rendering steering easier. Although this.bearing answers its purpose well when new after a certain time
• the constant hammering of the balls on their races, without any relative movement, as when running normally straight ahead, causes a series of depressions to be formed in the grooved washers, which destroys the
• frictionless properties of the bearing. Apparently, the old style of plain thrust washer is preferable, the slightly-increased friction being really not noticeable. War service has demonstrated that, for durability and reliability, the taper roller bearing is certainly superior to the ordinary bronze bush (floating or fixed) for road wheel journals. These bearings, of the Timken type, have beea run for long periods without showing any appreciable wear. On the contrary, the bronze bush is not only inferior from the mechanical point of view, but is extremely difficult to lubricate efficiently, . the arrangements usually provided for greasing being hopelessly inadequate for such an important and heavily-loaded bearing, and, what is ' more, its lubrication is sadly neglected by the average driver. A roller bearing once properly fitted and packed with lubricant requires practically no attention at all -and, if of sufficient size, wears indefinitely, so that it will -be somewhat surprising if all post-war vehicles are not so provided.
We have already referred to the advisability of large,diameter road wheels. The superiority of these is so manifest, both on account of smpothness of running and reduced wear and tear on all parts of the vehicle resulting from their use, that further comment seems needless.
The type of live axle in which the driving mechanism can he removed without dismantling the whole unit is now generally used and its superiority proved. Any other design which does .ncit give these advantages must, therefore, be considered out of date.
The axle casing should, preferably, be of such stout construction that a tie rod is not needed. This fitting is undesirable, for the same reason as mentioned above in connection with frame trusses. .
Springs should, preferably, be made up of a large number of comparatively thin leaves, avoiding unnecessary stiffness, whilst the camber should be small under load. The eyes should be bushed to receive the spring pins, which must have large-size lubricators capable of being easily turned without any tools. In many cases this latter requirement is met only on paper, with the inevitable result that lubrication of these parts is neglected, as are most things connected with the upkeep of a lorry which cannot be easily and quickly done.
In those cases where the drive is transmitted through the spriugs, special attention most be given to the secure fastening of the rear springs to their axle pads, as a loose nut here spells serious trouble
With regard to the steering gear generally, this component, including the connections to the leading
wheels, has, in some cases, proved to be rather too lightly Constructed, the gear itself not being mounted in a sufficiently rigid manner in the frame, and the ball or knuckle joints not being stout enough to withstand the shocks to which these parts are constantly being subjected when running on rough roads. The discussion of steering gear brings to mind an illustration of the fact that it is not always sound practice to accept criticism arising out of the uSe of war lorries at the Front as being necessarily aPplieable, in a general way, to what may be called normal circumstances.
Most French highways are constructed with a surface of " pave," or stone block paving, which occupies only the centre of the roadway, comprising, perhaps, one half of the total-width. This leaves on either side ‘,1,1 strip:of what is praetically unmetalled road. This form of conStruction served its purpose admirably in peace time, when all traffia could normally keep to the centre of the road, only leaving the " pave ' strip when passing other vehicles—a comparatively rare occurrence even on a long run. When, however. under the stress of war, traffic became congested, and. consisted largely of long convoys of motor lorries constantly passing in both directions, it became impossible, except on rare occasions, to keep to the centre ofthe road for any length of-time, and often convoys had to run for miles on end with their off-side wheels on the hard paving, whilst the near-side ones were forced on to the soft surface at the side.
The result of this kind of thing can easily be imagined, and the immediate effect on the vehicle is to cause a drag of the steering wheels towards the side. of the road. This tendency has, of course, to be corrected by the driver and the resulting arm strain on a i long run s considerable. • This fact has led to the criticism in some quarters that the leverage afforded by the steering wheels of standard lorries is insufficient ; but a moment's reflection will show that the above-clescribed eoliditions are entirely special, and are not likely to be met With in • other and normal circumstances. The normal arrangement of. steering gear, as laid
down in the W.D. Subsidiary Regulations,, allows of a
leverage of to 1 between steering wheel and road . wheels. This has been proved satisfactoryfor.all con. ditiorts of running, and does not involve undue strain on the driver.
We have briefly tolichee] upon only a few of the --yetis interesting amid instructive points of :Weak
'less Or imperfection which have been brought to light '
in the course of years of unparalleled trial for the motor vehicle. Other* will undoubtedly, occur to individual readers, but the point which we wish to em-. phasiz.e is that every complaint or criticism, no matter how apparently trivial, should be carefully, studied by those responsible for the design of Vehicle in questionnot to speak of those of their competitors !
Nothing is more certain than that no single type of lorry at present in use is perfect,' some, indeed, are. far lese so than others ; but the old proverb, " Experientia docet," was never more applicable than it is
to-day, and, if its teaching does not fall on deaf ears, the perfect commercial elicitor should be, at least, within our reach when the happy days of " recon struction " at last arrive. LIVE AXLE.